1. Field of the Invention
The invention relates to digitizers having a pen, and more particularly, to systems for detecting and indicating whether the writing tip of the pen is raised or lowered with respect to a writing surface of the digitizer.
2. Description of the Prior Art
Electronic digitizers which include a horizontal table having a grid of spaced conductors implanted in the X and Y directions beneath a writing surface supported by the table and also including a pen having a writing tip which can be raised above the writing surface or lowered to contact the writing surface are well known. For example, see U.S. Pat. Nos. 3,886,311 and 3,904,822.
Such digitizing systems are commonly utilized to enter digital data into a computer or to produce drawings or writings from data stored in a digital computer. In either case, it is frequently necessary for the pen to be lowered to make a mark on the writing surface, and then be raised and moved to another point above the writing surface and again lowered to make another mark on the writing surface spaced from the first mark. The pen assemblies of such systems may include an electromechanical device controlled by electronic circuitry and a digitizer for raising and lowering the pen tip in response to a control signal. In the course of entering data into a computer by means of a pen or making drawings by a computer controlled pen, some applications of digitizers require a pen tip to be raised from and lowered to a digitizer writing surface as many as several hundred thousand times per day. It is necessary that the electronic system controlling pen position be able to detect whether the pen tip is raised or lowered in order to prevent extraneous noise occurring when the pen tip is not contacting the writing surface from being interpreted as data. (It should be noted that even when the writing tip does not contact the writing surface, the pen tip and the underlying grid conductors may be sufficiently close to cause large magnitude signals to be detected, which large magnitude signals would cause data errors if no pen status signal were available to "gate out" such large magnitude signals.) In the past, a mechanical switch which is opened (or closed) when the pen is pressed against the writing surface and is closed (or opened) when the pen is raised has been utilized. However, due to the very large number of times that pens are raised and lowered in some applications, the reliability of such prior mechanical pen position status switches has been inadequate. Thus, although known mechanical pen position sensing switches have been economical, their reliability has been unacceptable and has caused a great deal of loss of digitizer operating time due to the fact that errors have been produced as result of such unreliability. The digitizers consequently have been inoperative during the time between discovery of a pen switch error and the time that a serviceman can repair the pen. Although present mechanical pen switches are initially economical, their costs in the forms of losses due to errors and system down time have been unacceptably high.
It is believed that optical switches including light emitting diodes, and photosensitive detectors have been utilized as pen status switches in digitizer systems. However, such devices are more expensive than presently used mechanical switches, since they require suitable encapsulation, means for producing a light-interrupting path, and bias circuitry for the light emitting diode and photosensitive detector circuitry.
Accordingly, it is an object of the invention to provide a low cost, yet highly reliable, system in a digitizer for indicating whether the writing tip of a digitizer pen is raised above a writing surface supported by the digitizer or is pressed against the writing surface.
Another object of the invention is to provide a highly reliable digitizer pen status indicating system which is readily compatible with circuitry utilized elsewhere in digitizers.
A novelty search directed to the subject matter of the invention uncovered U.S. Pat. No. 3,621,720, which discloses a linear variable differential transformer incorporated in a pen utilized to produce continuous signals representative of the pressure applied to a pen tip by a person utilizing the pen to write his signature. The continuous signals produced are amplified, and their magnitude is instantaneously traced out on a chart recorder and compared to a similar chart known to be correct for a particular signature to determine if the present water is forging the signature. Other patents deemed less relevant uncovered by the novelty search include U.S. Pat. Nos. 3,528,295, 3,235,790, 3,356,933, 3,376,533, 3,484,678, and 3,546,648.